FIG. 13 illustrates a moving-type radiation device of a conventional configuration. In this device, a radiation source 56 for irradiating radiation and a support column 52 supporting the radiation source 56 and extending in a vertical direction are provided on a hand-truck carriage. The support column 52 slidably supports the radiation source 56. The radiation source 56 is configured to be moved in the vertical direction while being supported by the support column 52. Such a device has electric assist, and is devised so that the operator can move easily without applying a strong force by hands.
Such a moving-type radiation device can be moved to a subject's room. By using this device, it becomes possible to perform radiation imaging of the subject without moving the subject as much as possible.
FIG. 14 schematically illustrates a mechanism for realizing the movements of the radiation source 56 in the vertical direction. The left side of FIG. 14 illustrates a typical mechanism of this device which uses a counter weight W having the same weight as the radiation source 56. That is, the device on the left side of FIG. 14 is provided with a wire having one end connected to the radiation source 56 and the other end connected to the counter weight W, and the wire is held by the pulley. In this way, the radiation source 56 and the counter weight W are related by a well bucket type mechanism. The load applied to one end side of the wire is equal to the load applied to the other end side thereof. Therefore, the radiation source 56 and the counter weight W are balanced with each other, and therefore they do not move by themselves. Then, when a force is applied to the radiation source 56, the radiation source 56 easily moves up and down.
The radiation source 56 has a considerable weight. The mechanism on the left side of FIG. 14 requires a counter weight W of the same weight as the radiation source 56. Therefore, when the radiation source 56 is installed on the device, the device increases in weight by the weight of the radiation source 56. Therefore, the moving-type radiation imaging device adopting this mechanism becomes considerably heavy. When the moving-type radiation imaging device is heavy, it is difficult to move it.
Therefore, a mechanism capable of reducing the weight of the moving-type radiation imaging device has been proposed (see Patent Document 1). The right side of FIG. 14 illustrates this new mechanism. This mechanism has a configuration in which the counter weight W on the left side of FIG. 14 is replaced with a tension spring. By removing the counter weight W, the weight of the device can be reduced by an amount equivalent to the weight of the radiation source 56.
In the mechanism shown on the right side of FIG. 14, the force transmitted by the radiation source 56 to the pulley is balanced with the tensile force of the tension spring. Therefore, the radiation source 56 and the tensile force of the spring are balanced with each other and radiation source 56 does not move by itself. Then, when a force is applied to the radiation source 56, the radiation source 56 easily moves up and down.
The tension spring has a property that the tensile force changes when the elongation of the spring changes. Therefore, in order to constantly balance the load of the radiation source 56 and the tensile force of the spring, some ingenuity is required. The pulley on the right side of FIG. 14 is a combination of a spiral winding drum and a cylindrical winding drum. In this case, the spiral winding drum denotes a fixed pulley having a round shaft whose radius changes according to the rotation angle. As the spiral, specifically, a spiral of Archimedes can be exemplified, but it is not always necessary to use a spiral of Archimedes. As the tension spring stretches, the tensile force increases, but the radius of the spiral decreases by that amount. To the contrary, when the tension spring contracts and the tensile force weakens, the radius of the spiral increases by that amount. As a result, the same torque is always applied to the pulley. Therefore, the radiation source 56 is moved up and down in the same behavior as when the counter weight W is used.